Film-based dual energy radiography

Abstract

Film packages for obtaining x-ray energy-difference images simultaneously by exposing a body to a broad energy spectrum x-ray beam. One package embodiment has two radiographic films joined at at least two edges to form a pocket in which a light-opaque sheet is inserted during the x-ray exposure. The package is disposed between front and rear x-ray intensifying screens during the exposure. The front screen on which the broad energy image is incident is excited to luminescence predominantly by x-ray photons in one energy band and the rear screen by photons in another band to thereby produce the difference images. The opaque sheet may contain x-ray filter material. A reflective sheet is substituted in the pocket for said opaque sheet during readout of the intensities of corresponding picture elements. Another film package embodiment is laminated and unitary and has a double-sided reflective sheet in the middle, a light-opaque coating on each reflective surface and a photosensitive emulsion on each coating. The unitary film package can be inserted between two screens, of the type described above, in a cassette during an exposure.

I claim: 
     
       1. A film package for forming on one radiographic film an image of an anatomical region corresponding to an image made by exposing the region to one x-ray energy spectral band and for forming on another film an image of the same region corresponding to an image made by exposing the region to another x-ray energy spectral band where both images are made simultaneously by exposing the region to a polyenergetic x-ray beam having a broad energy spectrum, said package being adapted for use with first and second parallel intensifying screens arranged in the path of the beam emergent from the region, which screens are excited to luminescence by the x-ray energy spectra that they absorb, said film package comprising: first and second parallel radiographic films fixedly joined together along at least two of their corresponding edges to form a pocket between the films, the photosensitive image recording surfaces of said first and second films being faced away from each other outside of the pocket for interfacing with said first and second intensifying screens, respectively, during the polyenergetic beam exposure, and   an x-ray transmissive and light-opaque sheet inserted in said pocket between the films for transmitting to said second film and second screen some of the x-radiation having energy spectra not absorbed by said first luminescent intensifying screen and first film and for preventing light from one intensifying screen from exposing the film interfaced with the other screen.   
     
     
       2. The film package according to claim 1 wherein said opaque sheet contains a material effective to filter out at least some of the lower energy x-ray photons within the broad spectrum that are not absorbed by the first intensifying screen so there will be proportionately more of the higher energy photons absorbed in the second screen than in the first screen. 
     
     
       3. The film package according to claim 1 wherein said opaque sheet contains at least one element having a K absorption edge within the broad x-ray energy spectrum for absorption of at least some of the relatively high energy photons within the broad spectrum that are not absorbed by the first screen so there will be proportionately more of the lower energy photons for absorption in the second screen. 
     
     
       4. The film package according to claim 1 wherein radiographic films are used that have a light response matched to the light spectrum or brightness of the respective intensifying screens with which the films interface. 
     
     
       5. The film package according to claim 1 wherein said first radiographic film on which the polyenergetic beam first impinges has a light response matched to a first screen of the type that contains an element having a K absorption edge for preferentially absorbing a substantial amount of relatively high energy photons within ssid broad x-ray spectrum. 
     
     
       6. The film package according to any of claims 3 or 5 wherein said element is selected from the group element having atomic numbers in the range of 53 to 74. 
     
     
       7. The film package according to any one of claims 3 or 5 wherein said element is gadolinium. 
     
     
       8. The film package according to claim 2 wherein said second film has a light response matched to a second intensifying screen of the type that contains calcium tungstate. 
     
     
       9. A device for recording two radiographic images on film where one of the images corresponds to an image of an anatomical region made by exposing the region to a different x-ray energy spectral band where both images are made simultaneously by exposing said region to a polyenergetic x-ray beam having a broad energy spectrum, said device being adapted for use between first and second parallel intensifying screens arranged in the path of the beam emergent from said region, which screens are excited to luminescense by the x-ray energy spectra they absorb, said device comprising: a substrate sheet coated on each of its sides with a light-reflective material,   a layer of photosensitive material on one side of the sheet and another layer of photosensitive material on the other side of the sheet and said layers facing away from each other for the layer on the one side to interface with said first intensifying screen and said layer on the otherside to interface with said second intensifying screen during exposure to said polyenergetic emergent x-ray beam, said coating and layer bearing substrate sheet transmitting to said second screen at least some of the emergent x-radiation that impinges on the first intensifying screen but is not absorbed by said first intensifying screen and preventing light emitted by one screen from exposing the photosensitive material interfaced with the other screen.   
     
     
       10. The device according to claim 9 including a coating of material that is opaque to light interposed between said reflective coatings and said photosensitive layers, said opaque material coating being of a type that is removed or made non-opaque by development of said photosensitive layers so the residual images in the photosensitive layers have the reflective coatings behind them to facilitate readout of the individual images. 
     
     
       11. The device according to any one of claims 9 or 10 wherein said photosensitive layer on the one side of the substrate sheet has a response to light matching the spectrum and brightness of the light emitted by said first intensifying screen and said photosensitive layer on the other side of the substrate has a response to light matching the spectrum and brightness of the light emitted by the second intensifying screen. 
     
     
       12. The device according to claim 11 wherein said first photosensitive layer records an image corresponding to the light spectrum emitted by the first intensifying screen by having said screen comprised of a material including an element having a K absorption edge within said broad x-ray spectrum so that said element will absorb more of the high energy x-ray spectra. 
     
     
       13. The device according to claim 12 wherein said element is selected from the group of elements having atomic numbers in the range of 53 to 74. 
     
     
       14. The device according to any one of claims 9 or 10, wherein said reflective layer is comprised of an x-ray filtering material. 
     
     
       15. The device according to claim 11 wherein said light-reflective layer is comprised of an x-ray filtering material. 
     
     
       16. The device according to claim 14 wherein said x-ray filtering material includes an element having a K absorption edge within the broad x-ray spectrum to thereby enhance filtering out of high energy x-ray spectra. 
     
     
       17. The device according to claim 15 wherein said x-ray filtering material includes an element having a K absorption edge within the broad x-ray spectrum to thereby enhance filtering out of high energy x-ray spectra. 
     
     
       18. The device according to claim 14 wherein said filtering material includes an element selected from the group of elements having atomic numbers in the range of 53 to 74. 
     
     
       19. The device according to claim 14 wherein said filtering material includes gadolinium.